Such a system is described in United States patent application publication 2009/0202119. This system comprises a surgical microscope, which is designed for observing the surgical region using fluorescence light. The surgical microscope comprises an illumination apparatus with a xenon lamp as light source. There is a narrowband filter in the illumination apparatus. This filter is transmissive for light in the wavelength region between 390 nm and 410 nm. Using light of this wavelength, it is possible to excite the dye protoporphyrin IX (PPIX) to fluoresce. The dye PPIX can be selectively accumulated in pathologically changed tumor-diseased cells. The surgical microscope comprises a detection apparatus, which contains a color camera. This color camera can be used to capture the fluorescence light from PPIX. The color camera is connected to a computer unit for image processing. In this computer unit, the signal from the color camera corresponding to the red color channel is converted into a grayscale-value image. Subsequently, the intensity of each pixel in the grayscale-value image is compared to a threshold. Boundary intensity lines are then calculated from this comparison. These boundary intensity lines are displayed together with the surgical area on a visualization apparatus having a screen. The boundary intensity lines surround tumor-diseased tissue. As a result, the boundary intensity lines make it easier for an operator to identify tumor tissue in a surgical region.
Experience has shown that the system described in United States patent application publication 2009/0202119 cannot always reliably visualize the position of the edge regions of tissue afflicted by tumor using the boundary intensity lines.
“R. Ishihara et al., Quantitative Spectroscopic Analysis of 5-ALA-induced Protoporphyrin IX Fluorescence Intensity in Diffusely Infiltrating Astrocytomas, Neuro Med Chir (Tokyo) 47, 53 (2007)” has disclosed that it is possible to distinguish healthy regions from regions afflicted by tumor in tissue, enriched with the substance PPIX, of the human brain by evaluating intensity peaks which occur at characteristic wavelengths in the fluorescence spectrum of protoporphyrin IX. This method requires a high degree of measurement and computational complexity.